Experiments on rats show that following injection of S35-cysteine into the supraoptic nucleus, labeled oxytocin, vasopressin, and neurophysin appear first in the nucleus, then in the median eminence (1 hour), and finally in the posterior pituitary (greater than 1.5 hour). The peptides and neurophysin are synthesized first as a protein precursor(s) (mw, 20,000 daltons) and the enzymatic degradation of the precursor occurs intra-granularly during axonal transport. Two precursors, one for vasopressin and one for oxytocin have been identified immunologically. Rats with heredity diabetes insipidus do not synthesize the vasopressin precursor. Several unknown peptides also transported to the pituitary from the hypothalamus have been discovered and may represent new hormones.